As for the magnetic core (core material) of an electromagnetic part (e.g., motor) used with an alternating current, a magnetic core obtained by laminating an electromagnetic steel sheet, an electrical iron sheet or the like has been heretofore used. However, in recent years, a powder core produced by compacting a soft magnetic iron-base powder and annealing this for strain relief is put into use. Compacting of an iron-base powder brings about a high shape latitude and enables easy production of even a three-dimensionally shaped magnetic core. Accordingly, as compared with a magnetic core produced by laminating an electromagnetic steel sheet, an electronic iron sheet or the like, downsizing or lightweighting becomes possible. Also, after compacting, strain-relief annealing is performed, whereby the strain introduced at the production or compacting of the raw material powder can be relieved and the core loss, particularly hysteresis loss, can be reduced.
The powder core produced by compacting an iron-base powder exhibits good electromagnetic conversion property in a high frequency band of, for example, 1 kHz or more, but in the drive conditions under which the motor is generally working [for example, at a drive frequency of several hundreds of Hz to 1 kHz and a drive magnetic flux of 1 T (tesla) or more], the electromagnetic conversion property is likely to deteriorate. When the change of magnetic flux inside of the material is in a region of not involving a relaxation phenomenon (e.g., magnetic resonance), the deterioration of electromagnetic conversion property [that is, energy loss (core loss) at the magnetic conversion] is known to be expressed by the sum of hysteresis loss and eddy-current loss (see, for example, Non-Patent Document 1).
Out of these losses, the hysteresis loss is considered to correspond to the area of a B—H (magnetic flux density-magnetic field) curve. The factor affecting the shape of this B—H curve and governing the hysteresis loss includes the coercivity (loop width of B—H curve), the maximum magnetic flux density and the like of the powder core. In other words, the hysteresis loss is proportional to the coercivity and therefore, for reducing the hysteresis loss, this may be attained by reducing the coercivity.
On the other hand, the eddy-current loss is a joule loss of the induced current associated with an electromotive force generated by electromagnetic induction according to a change in the magnetic field. This eddy-current loss is considered to be proportional to the change rate of magnetic field, that is, the square of frequency, and as the electric resistance of the powder core is smaller or as the range in which an eddy current flows is larger, the eddy-current loss becomes larger. The eddy current is roughly classified into an intraparticle eddy current that flows in individual iron-base powder particles and an interparticle eddy current that flows across between iron-base powder particles. Accordingly, when electrical insulation among individual iron-base powders is complete, the interparticle eddy current is not generated and only the intraparticle eddy current flows, so that the eddy-current loss can be reduced.
Meanwhile, with respect to deterioration of the electromagnetic conversion property, in a low-frequency band at which the motor is generally working (for example, from several hundreds of Hz to 1 kHz), the hysteresis loss is more governing than the eddy-current loss and it is demanded to reduce the hysteresis loss.
In regard to the technique for reducing the hysteresis loss, Non-Patent document 1 discloses a technique aiming at characteristic improvements while paying attention to achieving a low coercivity of a magnetic powder by the elevation of purity and the reduction in intraparticle strain, achieving a high density of the green compact, achieving high electrical resistance, and enhancing the heat resistance by the improvement of insulating film. However, this technique lacks general-purpose applicability, because an iron-base powder made to have a high purity by reducing the amount of impurities inevitably contained in the iron-base powder needs to be used and an iron-base powder commercially available in general cannot be used.
On the other hand, Patent Document 1 proposes a pure iron powder for powder metallurgy, which is a coarse crystal grain having a particle size construction such that, in terms of the sieve weight ratio (%) determined using a sieve defined in JIS Z8801, a portion passed through a −60/+83 mesh accounts for 5% or less, a portion through a −83/+100 mesh accounts for 4% or more and 10% or less, a portion passed through a −100/+140 mesh accounts for 10% or more and 25% or less, and a portion passed through a 330 mesh accounts for 10% or more and 30% or less, where the average crystal grain size of the portion passed through a −60/+200 mesh is 6.0 or less according to a measurement method for ferrite crystal grain size defined in JIS. In Patent Document 1, it is indicated that when the ferrite crystal grain size is increased, the magnetic field is reduced for the soft magnetic property and this is advantageous from the standpoint of deterring the formation of a magnetic domain as well as in view of internal loss. However, in Patent Document 1, a coarse particle failed in passing through a 60 mesh (a sieve having a sieve opening of 250 μm) is not used so as to avoid impairment of the homogeneity of the green compact and generation of a defect in terms of strength.
Also, Patent Document 2 describes a technique of setting, in the cut surface of a metal powder particle, the number of crystal grains in one metal powder particle to 10 or less on average and indicates that reduction in the number of crystal gains may be attained by a method of heating the metal powder particle at a high temperature in a heating atmosphere. However, according to the study by the present inventors on the technique disclosed in Patent Document 2, there is a case where even when the number of crystal grains in individual metal powder particles is controlled, the magnetic permeability of the powder core is not improved and the hysteresis loss cannot be reduced. Accordingly, the core loss of the powder core is not sufficiently improved in some cases.
Non-Patent Document 1: SEI Technical Review, No. 166, pp. 1-6, issued by Sumitomo Electric Industries, Ltd. (March, 2005)
Patent Document 1: JP-A-6-2007
Patent Document 2: JP-A-2002-121601